Random access memory (RAM) falls generally into two categories, namely, static and dynamic. Both types of RAM are made up of an array of addressable storage cells, each one storing a bit of information (or multi-bit information). In static RAM (SRAM), the storage unit within the cell is typically a bi-stable flip-flop and the state of the flip-flop indicates whether the cell stores a zero or a one. In dynamic RAM (DRAM) the storage unit is typically an integrated circuit (IC) capacitor and the charge on the capacitor indicates whether the stored value is either a zero or a one. In the case of DRAM's, however, capacitors gradually lose charge and thus it is necessary to “refresh” the stored data as often as every few milliseconds. The refreshing operation is performed by first reading the value in the cell and then writing it back. This is done for every cell in the array and it typically requires complicated control circuitry. Storage-Class memory has lower read/write speed than DRAM, but a much longer retention time, and therefore it can keep data even without power supply. A common storage-class memory is flash memory based on floating gate transistor and/or charge-trap transistor.
Due to its fast switching speed and long retention, FeFET (Ferroelectric-gated Field-Effect Transistor) can perform the function of DRAM and/or one or more Flash memory cells. FeFET-based DRAM (FeDRAM) has several advantages over the conventional DRAM including, for example, that FeDRAM: (1) is made of a capacitor-less single-transistor structure, which is more compact, easier to fabricate, and more scalable; (2) has much longer retention that enables low refresh frequencies, and therefore much lower power consumption; and (3) has non-destructive read-out schemes, and therefore does not require a write operation to recover data after every read operation. The recent discovery of HfO2-based ferroelectric film has overcome some limitations of FeFET that use conventional ferroelectric materials, making FeDRAM much more viable. In addition, FeFET based flash memory cells are also possible with HfO2-based ferroelectric, which has much faster switching speed and better endurance than conventional floating gate and charge-trap based flash.
Three-dimensional (3D) structures can potentially increase the memory density, shorten connection paths, and reduce the cost per memory bit for DRAM and Storage-Class Memory. Recently, there has been some developments with 3D NAND structures, which are specifically designed for high density storage application without random access capability. For random access memory capability, 3D memory arrays with NOR connections have been reported.